He became a contributor to the satirical magazine Punch, who first published his work in 1952,[2] beginning a 25-year relationship that resulted in more than 1,500 cartoons, of which 60 were used as front covers.[2] He also worked as political cartoonist for the News Chronicle from 1956 until the paper closed in 1960.[3]

His first collection of cartoons, Angels on Horseback, was published in 1957.[2]

Known to many only as Thelwell, he found his true comic niche with Pony Club girls and their comic ponies, a subject for which he became best-known, and which led to a cartoon strip about such a pair, Penelope and Kipper. He also illustrated Chicko in the British boys' comicEagle.

For the last quarter of a century of his life he lived in the Test Valley at Timsbury, near Romsey, gradually restoring a farm house and landscaping the grounds which gave rise in 1978 to his first factual book, A Plank Bridge by a Pool,[4] which detailed the first two lakes he dug there. A third lake was later featured on the BBC’s South Today programme. Written much earlier, but published three years later, A Millstone Round My Neck described his experiences in re-building a Cornish water mill (Addicroft Mill at Liskeard, which he called Penruin), that was sold before the book was published. He always loved old buildings, and in his autobiography, Wrestling with a Pencil, wrote about his joy in the beauty of old cottages.